The US electrical energy generating capacity is 1,088 GW and the storage capacity is 22 GW. Most of this storage capacity (99%) is provided by “pumped hydro” energy storage systems. Such systems store energy in the form of potential energy during off-peak hours and convert the stored energy to electricity during peak hours. However, such systems have several distinct disadvantages: (1) they require a large volume to store energy leading to a low volumetric energy density; (2) they are highly dependent on geography or geology and provides little siting flexibility; (3) has low energy efficiency because storage and recovery of electrical energy relies conversion between electrical energy and mechanical energy (i.e., incurs frictional losses); and (4) they can be slow to react to changing energy demand conditions to store and provide electricity in the minutes to hour time scale.